The present invention relates to improvements in and relating to the method of manufacturing line pipes for use in cold districts where high strength and high toughness are required as well as high-strength steel pipes which are required to retain a satisfactory toughness at low temperatures. More particularly, the present invention relates to an improved method of manufacturing such steel pipes wherein the steel plate which has been formed by such forming process as UOE or spiral process is arc welded to produce the desired steel pipe.
As pipe line installations installed in the cold districts for conveying petroleum, natural gas and the like have recently become increasing-large, there has been an increasing demand for thick, high-strength steel pipes which have excellent strength and retain excellent toughness at low temperatures. Generally, it has been customary to manufacture the large-diameter steel pipes for such applications on a mass production basis by using such process as the UOE, spiral, cage forming, bending roll forming or the like. It is needless to say that these pipe manufacturing processes in which the welding is accomplished after the plates have been formed, naturally have a very great demand for higher operating speeds and higher operating efficiency. In the past, the tandem sequence type, large heat input submerged arc welding process with one pass per electrode on each side of the seam has been employed for the regular welding of the tubes prepared by the abovementioned forming processes. In that case, while new processes such as the Q - T treatment of pipe have been proposed to meet the above-mentioned requirements, there are many problems which have not been solved as yet. One of these unsolved problems is how to provide a satisfactory strength and toughness in a steel plate constituting the base metal for welded pipe. For this purpose, a so-called controlled rolling (CR) process has been proposed and practiced in which the total rolling reduction during the rolling operations effected at relatively low temperatures of below 900.degree. C is increased. A disadvantage of this process is that there is a limit to the total rolling reduction at such low temperatures which involve an increased deformation resistance and therefore the effects of the CR process on thick steel plates cannot be as high as desired. Consequently, the thick steel plate must be subjected to additional heat treatments such as quenching and tempering to produce the desired high strength and toughness. However, it is self-evident that not only these properties are essential in the base metal but also these properties in the weld zone must be satisfactorily high. In the case of a line pipe designed for conveying natural gas from a cold district, the requirements for low temperature toughness in both the base metal and weld zone are extremely severe, since such line pipe is designed for conveying the natural gas under high pressure at high speeds. Particularly, the hardness of the weld zone is limited to low values for the purpose of preventing the danger of stress corrosion cracking of the pipe due to the sulfides contained in the natural gas. On the other hand, it is also self-evident that the addition or increase in the amount of alloying elements is necessary since the manufacture of thick, high-strength high-toughness steel plate requires the controlled rolling or heat treatments as well. For instance, to improve the hardenability of the material, it is necessary to increase the Mn content or add such elements as Cr and Mo. These elements are essential alloying elements for providing the desired high toughness in the base metal. However, while the base metal is satisfactory in terms of its properties, the properties, particularly toughness in the weld zone is entirely different from that of the base metal. In other words, with the presently available large heat input, submerged arc welding process, it is necessary to increase the heat input as the thickness of the steel pipe increases in order to meet the desired welding efficiency with one pass per electrode on each side of the seam. For instance, where the steel is welded with this process using as high a heat input as 65,000 Joule/cm. the properties, particularly low temperature toughness in the heat affected zone (hereinafter simply referred to as H.A.Z) adjacent to the bond or fusion line of the weld zone is inevitably deteriorated. The present tendency is toward preventing and controlling the deterioration of the the properties in weld zone by changing the chemical composition of a steel plate or base metal. However, it is clear that the degree of deterioration in properties of the weld zone increases as higher quality steel is used as the base metal. As regards the maximum hardness of the H.A.Z, practically all of the alloying elements required for providing a high degree of toughness in the base metal tend to increase the maximum hardness of the H.A.Z. Thus, while it has been considered imperative to use steel plates having higher alloy chemical compositions as the base metals to obtain the desired high-strength, high-toughness thick steel pipes, the presently available welding processes bring about relative deterioration of the properties in the H.A.Z. and increase its hardness, thus making it impossible to obtain the desired steel pipe.
To avoid the above-mentioned problem of deterioration in toughness, so far as the presently available submerged arc welding process is used, there is no other way but to control the welding heat input and this controlled heat input inevitably necessitates the deposition of the weld metal in multiple layers. The use of such multi-layer submerged arc welding with a reduced heat input requires the flux removing operation upon completion of each pass with a resulting deterioration of the welding efficiency. Another disadvantage is the necessity of using a high basic flux for ensuring the desired toughness at low temperatures, since such high basic flux frequently gives rise to welding defects. Even if the steel plate is thin permitting the deposition of the weld metal with one pass per electrode on each side of the seam, the reduced heat input results in an increased rate of cooling of the heat affected zone and the maximum hardness of the H.A.Z is inevitably increased. This rising tendency in the maximum hardness tends to become marked as the amounts of alloying elements added to the steel plate or base metal are increased. In view of these circumstances, it may safely be said that in view of alloy compositions of steel plates or base metals, the submerged arc welding process is not suitable as the process of welding large-diameter thick steel pipes for the manufacture of very low temperature line pipes. Particularly, in the case of steel plates of over 12 mm thick, it may be impossible to produce steel pipes having no such defects by using the submerged arc welding process.
Although the use of other low heat input welding process, e.g., the MIG welding process or CO.sub.2 gas shielded welding process may be considered, these processes are so designed that the welding is accomplished in an atmosphere consisting principally of argon or carbon dioxide gas by operating a small-diameter wire electrode of less than 2.0 mm.phi. at welding currents lower than 500 amp. In these cases, while the problem of deteriorated low temperature toughness in the welding heat affected zone due to the low heat input is overcome, the welding efficiency is reduced and the maximum hardness of the H.A.Z is increased as in the case of the low heat input, submerged arc welding. To prevent the lowering of welding efficiency, a welding process has been proposed in which a plurality of small-diameter wire electrodes are arranged in a straight row along the direction of welding to accomplish the continuous welding. However, it has been found that the improved welding efficiency provided by this process is also far short of the drastic improvement of the welding efficiency and the deficiencies on the maximum hardness remain almost unsolved. In other words, it should be concluded that none of the presently available processes for making welded pipes can be used as the practical welding process for making steel pipes having high strength and high toughness in both the base metal and the weld zone.